1. Field of the Invention
The present invention relates to an injection molding apparatus, and in particular to an injection molding apparatus for forming a hollow molded product with plural resin materials such as two-color injection molding or sandwich molding.
2. Related Background Art
Injection molding of hollow products such as buckets, boxes or bottles with plural resin materials is known as two-color molding.
So-called sandwich injection molding machines for injecting a core layer resin into a skin layer resin for increasing the mechanical strength of the hollow product is already known in various forms.
When the sandwich injection molding technology is used for the production of optical instruments such as components of a lens tube of an interchangeable camera lens, the following conditions should be satisfied:
(1) the injection process is short and can be completed quickly;
(2) plural resin materials are not mutually mixed; and
(3) the core layer is uniformly injected into the skin layer in any cross section of the molded product.
In a conventional two-color injection molding machine, a fixed plate is connected to first and second injecting cylinders; a mold plate having a molding gap and a runner plate is positioned between said fixed plate and mold plate, and sprues are provided for injecting resin into the fixed plate. After injecting a first resin, a metal mold unit composed of the fixed plate, runner plate and mold plate is moved by rotation or displacement with respect to the injecting cylinders so as to expand the gap provided in the mold plate, and a second resin is infected to change the specie of resin flowing into the sprues of the fixed plate.
However, in such conventional structure, the first resin has already cooled and solidified when the second resin is injected because the rotational movement or displacement takes time, so that the second resin becomes attached around the first resin or to a part thereof. Consequently, insufficient adhesion strength between the first and second resins is usually supplemented by mechanical means such as a flange. Also, since the second resin is injected after the first resin is cooled, the cooled first resin is subjected to an external force due to the injecting pressure of the second resin, and, for this reason, it has been considered difficult to obtain molded products of a high precision.
Because of the above-mentioned reasons, it has been considered impossible to produce optical instruments, such as lens tube components of an interchangeable camera lens, because of the required high precision.
There has been proposed a sandwich molding machine for injecting first resin and then injecting second resin while said first resin is still in melted state, thereby surrounding the second resin with the first resin, as disclosed in the U.S. Pat. Nos. 3,809,519, 3,599,290 and 3,733,156. However these patents switch the first and second resins with a rotary or sliding valve which requires a certain time, so that the first resin is cooled to significantly increase the pressure loss at the injection of the second resin. It is therefore difficult to maintain the precision of the molded product. In addition the flow characteristic of the first or second resin is not stabilized because of the first or second resin remaining in, in front of, or behind the valve, so that the ratio of the first and second resins fluctuates and the molding precision is difficult to maintain.
In consideration of the aforementioned drawbacks the present inventors have reached the present invention through investigations with the objectives that:
(1) the injection process can be completed within a short time;
(2) plural resin materials are not mutually mixed;
(3) ratio of the skin layer and the core layer is uniform in various cross sections of the molded product;
(4) the resin material used in an injection process does not remain in a further injection process;
(5) the resin material should not remain except in the injecting cylinders;
(6) the pressure loss in the resin flow path should be small at the injection;
(7) no switching means should be present in the resin flow path;
(8) molded products should be continuously and stably obtained with a high precision; and
(9) the resin material should be poured uniformly into plural runners or gates, as employed.